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Carbonylation of Methanol on MetalЦAcid Zeolites Evidence for a Mechanism Involving a Multisite Active Center.

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Zuschriften
DOI: 10.1002/ange.200700029
Carbonylation
Carbonylation of Methanol on Metal?Acid Zeolites: Evidence for a
Mechanism Involving a Multisite Active Center**
Teresa Blasco, Mercedes Boronat, Patricia Concepcin, Avelino Corma,* David Law, and
Jose Alejandro Vidal-Moya
Carbonylation of alcohols and olefins in the presence of water
with CO to produce acids and acetates is carried out at high
pressure in the presence of strong Br鴑sted and Lewis acid
catalysts.[1] Organometallic complexes containing Rh or Ir
together with iodide compounds are able to catalyze such
reactions at low temperatures and atmospheric pressure,[2, 3]
and they are now used for the synthesis of acetic acid from
methanol and CO. However, the finding that zeolites are able
also to catalyze the carbonylation of alcohols at low temperatures and atmospheric pressure has renewed interest in the
use of solid catalysts, and more specifically zeolites, for
developing a halide-free carbonylation catalyst.[4?7]
Concerning the reaction mechanism with zeolites, it has
been found that the alcohol adsorbs on Br鴑sted acid sites to
form a methoxy species, which reacts with CO to form an
acylium cation that can be quenched by H2O to give the
corresponding acid.[4, 6, 8?10] The limiting step of the reaction is
believed to be the attack of the methoxy species by CO to
form an acylium cation type species. Very interesting results
were recently presented for the reaction of dimethyl ether
(DME) with CO on H-Mordenite in the absence of H2O; the
reaction revealed excellent selectivity to give methyl acetate
at low temperatures (423?463 K).[11] A reaction mechanism in
which the limiting step of the reaction involved attack of CO
at a methoxy group was proposed on the basis of kinetic
experiments[11] and was confirmed later by NMR spectroscopy results.[12] As a result of the high selectivity for methyl
acetate when using DME under H2O-free reaction conditions,[12] this work represents an important step forward.
Nevertheless, it would be preferable, from a process point of
view, to use methanol instead of DME as feed. However,
when methanol was used, conversion was much lower and
[*] Dr. T. Blasco, Dr. M. Boronat, Dr. P. Concepci!n, Prof. A. Corma,
Dr. J. A. Vidal-Moya
Instituto de Tecnolog+a Qu+mica, UPV-CSIC
Universidad Polit0cnica de Valencia
Avda. de los Naranjos s/n, 46022 Valencia (Spain)
Fax: (+ 34) 96-387-7809
E-mail: acorma@itq.upv.es
Dr. D. Law
BP Chemicals
Hull Research & Technology Centre
Saltend, Hull HU12 8DS (UK)
[**] The authors thank BP Chemicals for financial support and for
permission to publish these results. J.A.V.-M. thanks the CSIC (grant
I3P). Thanks also to the CICYT (project MAT2006-14274-C02-01) for
financial support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
4012
higher reaction temperatures were required with the corresponding formation of hydrocarbons.[7] As the controlling step
of the reaction is the attack of CO at the adsorbed methoxy
species, there is a possibility to increase the rate of reaction
through a bifunctional catalyst in which the methoxy groups
would be formed on a Br鴑sted acid site of the zeolite while
CO would be activated at a metal site located in a next near
neighbor aluminum framework.
We report here a detailed mechanistic study of methanol
carbonylation on H-Mordenite and Cu-H-Mordenite that was
carried out by using IR operando spectroscopy and ?in situ?
magic-angle spinning (MAS) NMR spectroscopy. We
observed the formation of intermediate methoxy and acylium
species and found that Cu not only activates CO under
reaction conditions but also absorbs DME preferentially to
H2O and methanol. This preferential adsorption favors the
attack of DME at the acylium cation in the case of Cu-HMordenite to give methyl acetate as the primary product.
Meanwhile, in the case of H-Mordenite, water adsorption
prevails and acetic acid is produced as the primary and
predominant product. It is remarkable that under the
reactions conditions studied here (200 8C?240 8C), the formation of hydrocarbons was not observed while almost total
conversion of methanol and DME occurred.
The IR spectrum of Cu-H-Mordenite activated with CO
at 350 8C (see Experimental Section) shows an intense and
narrow band at 2159 cm 1 assigned to a Cu+-monocarbonyl
species.[13, 14] This band shifts to 2129 cm 1 after the coadsorption of methanol/CO at 100 8C, and the shift is
completely reversed upon methanol desorption, suggesting
that methanol, an electron-donor group, adsorbs onto the
Cu+-CO complexes (see Figure 1 in the Supporting Information).
The IR spectra recorded after heating the methanol/CO
mixture at 200 8C over H-mordenite and Cu-H-Mordenite
show the presence of dimethyl ether as well as the formation
of methoxy groups over Br鴑sted acid sites (2975 cm 1) and
AlOH (2969 cm 1) and silanol (2956 cm 1) groups.[15, 16] The
n(C H) stretching vibration band of dimethyl ether appears
at 2834 cm 1 in H-Mordenite and at 2840 cm 1 in Cu-HMordenite, suggesting that this molecule interacts with Cu+
ions, as supported by the appearance of a second band at
2133 cm 1 in the carbonyl stretching vibration region (see
Figure 2 in the Supporting Information). Therefore, besides
monocarbonyl Cu+ species (2159 cm 1), dimethyl ether (electron donor) adsorbed on Cu+-CO species is formed. For both
Cu-H-Mordenite and H-Mordenite, a band corresponding to
adsorbed water appears at 1630 cm 1 upon cooling down the
2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2007, 119, 4012 ?4015
Angewandte
Chemie
sample to room temperature, which suggests that H2O
remains in the gas phase at 200 8C.
To detect possible reaction intermediates over H-Mordenite and Cu-H-Mordenite, time-resolved IR spectra were
recorded at 240 8C (see Figure 1). For H-Mordenite, the band
Figure 1. Time-resolved FTIR spectra of a) H-Mordenite at 6, 17, 23,
and 49 min and b) Cu-H-Mordenite at 1, 5, 15, and 40 min, after CO/
MeOH co-adsorption at 240 8C. The insets show the 4000?2500 cm 1
IR region.
at 1458 cm 1 from the CH deformation vibration of dimethyl
ether decreases in intensity, while new bands at 1490, 1427,
1387, and 1366 cm 1 increase with time. These bands together
with that at 3266 cm 1, which is characteristic of acidic
compounds, (see inset of Figure 1 a) indicate the formation
of acetic acid. In Cu-H-Mordenite, the intensities of the bands
for the Cu+-carbonyl complexes, both isolated (2159 cm 1)
and co-adsorbed with DME (2133 cm 1), decrease with
reaction time, while the intensities of two bands at 1496 and
1387 cm 1 associated with COO groups (acetate) and a weak
band at 3266 cm 1 for acetic acid increase with reaction time
(see Figure 1 b). We note the presence of a band (C=O) at
1685 cm 1 which disappears after 90 minutes reaction (Figure 1 b). This intermediate must be formed by reaction of
carbon monoxide (CO) with a carbocation, which probably
forms from the dissociation of DME or from methoxy groups
of the zeolite surface.
Up to now and from the IR operando spectroscopy study,
we can conclude that the carbonylation reaction proceeds
differently on H-Mordenite and Cu-H-Mordenite zeolites.
Angew. Chem. 2007, 119, 4012 ?4015
CO forms monocarbonyl Cu+-CO complexes up to 350 8C on
the Cu-H-Mordenite zeolite. At 200 8C, DME, methoxy
species, and water are formed on both zeolites, but DME is
co-adsorbed on the Cu+-CO sites in Cu-H-Mordenite, preferentially to H2O. The CO group involved in the monocarbonyl Cu+-CO complex can react with close methoxy
groups to form a CH3CO+ reactive intermediate, which we
were not able to detect in the H-Mordenite zeolite probably
because of the lower concentration. DME that is stabilized on
Cu-H-Mordenite reacts with the acylium cation formed on
the acid sites of the zeolite leading to the preferential
formation of acetate. Meanwhile, formation of the acid is
preferred on the H-Mordenite zeolite.
The results and conclusions obtained by IR spectroscopy
are fully supported by the in situ NMR experiments, carried
out using labeled molecules. To identify the species strongly
bonded to the zeolites, we recorded 13C NMR spectra after
the reaction of 13CO/13CH3OH at 180 8C over H-Mordenite
and Cu-H-Mordenite using a low reactant concentration
(13CH3OH/Al = 0.25, 13CO/13CH3OH = 3; see Figure 3 in the
Supporting Information). For both zeolites, the simulated
spectra provide evidence for the presence of dimethyl ether
with side-on and end-on configurations, methoxy species
formed on Br鴑sted hydroxy and silanol groups, and methanol adsorbed end-on and side-on.
The 13C MAS NMR spectra of the CO/13CH3OH/HMordenite (molar ratios: 13CO/13CH3OH = 3, 13CH3OH/Al =
0.5) system heated at 200 8C up to 15 min consist of a very
intense signal for dimethyl ether at d = 59.6 ppm and a weaker
signal for methanol at d = 50.7 ppm. After heating for 15 min,
a new signal emerges at approximately d = 20 ppm, which is
assigned to the methyl group involved in 13CH3-13COO acetyl
groups. This signal gradually increases with reaction time,
while those for methanol and dimethyl ether decrease (see
Figure 4 in the Supporting Information). Inspection of the
NMR region corresponding to the methyl-acetyls (13CH3COO ) reveals the presence of two components at d = 20.6
and 19.2 ppm, which can be assigned to acetic acid (13CH313
COOH) and methyl acetate (13CH3-13COO13CH3), respectively. The formation of the latter product is confirmed by the
appearance of a peak at d = 53.4 ppm, from the methoxy
group of methyl acetate (13CH3-13COO13CH3), which becomes
more pronounced at long reaction times (see Figure 4 in the
Supporting Information).
Although the same species are formed on Cu-H-Mordenite, the 13C NMR signal for the methyl group from 13CH313
COO appears after 5 min at 200 8C. Moreover, the spectra
simulation using two individual lines for 13CH3-13COOH (d =
20.6 ppm) and 13CH3-13COO13CH3 (d = 19.2 ppm) indicates
that the acid predominates on H-Mordenite while the acetate
predominates on Cu-H-Mordenite, in agreement with the IR
results (see Figure 5 in the Supporting Information).
Concerning the carbonyl region of the 13C MAS NMR
spectra, besides the presence of 13CO within the zeolite
channels, the Cu-H-Mordenite sample shows an additional
broad band at d = 171 ppm which can be assigned to 13CO
molecules adsorbed over Cu+ cations[17] (see Figure 6 in the
Supporting Information). By taking into account previous
results and the fact that CO and DME are preferentially
2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4013
Zuschriften
adsorbed on Cu+, we should expect a higher rate of reaction
for the formation of acetyls with Cu-H-Mordenite than with
H-Mordenite during carbonylation of methanol. Thus, the
kinetics of the process was followed by ?in situ? MAS NMR,
and the results clearly show more than a threefold increase in
the reaction rate when Cu is introduced (Figure 2).
Figure 2. Conversion of 13CH3OH to acetyls in the 13CO/13CH3OH
reaction at 200 8C over H-Mordenite (H-MOR) and Cu-Mordenite (CuH-MOR), as measured by in situ 13C MAS NMR spectroscopy.
In conclusion, we have confirmed the reaction mechanisms proposed for the carbonylation of methanol on protonic
zeolites[8?10] by detecting the intermediate species formed on
the catalyst surface by means of IR operando spectroscopy
and ?in situ? MAS NMR spectroscopy. We have also shown
that a different reaction mechanism occurs on a metal zeolite
(Cu-H-Mordenite). Thus, while acetic acid is formed as a
primary acetyl product when reacting methanol on HMordenite, methyl acetate is formed in the case of Cu-HMordenite. It is shown that in the case of Cu-H-Mordenite,
the reaction rate is much higher than with H-Mordenite. The
above results can be explained on the basis of a reaction
mechanism in Cu-H-Mordenite that involves an active site
composed of two neighboring sites, one bridged hydroxy and
a neighboring Cu+. The Br鴑sted acid site is responsible for
activating the methanol (methoxy), while Cu+ can activate the
CO and adsorbs preferentially the DME compared to
methanol and water under our reaction conditions. A
mechanism is proposed in Scheme 1 for the carbonylation
of methanol on a purely acid zeolite (H-Mordenite) and on a
metal-containing acid zeolite (Cu-H-Mordenite).
Scheme 1. A potential reaction mechanism for carbonylation of methanol with CO on H-Mordenite (H-MOR) and Cu-H-Mordenite (Cu-HMOR).
for 12 h. Finally, the material obtained was calcined at 500 8C for 2 h.
The amount of Cu in the sample was 2.5 % by weight.
FTIR experiments were performed with a Biorad FTS-40 A
spectrometer using an IR flow quartz cell connected to a vacuum line
with gas-dosing possibilities. Prior to adsorption experiments, HMordenite and Cu-H-Mordenite zeolites were activated at 350 8C in
N2 flow (20 mL min 1) for 2 h, followed by two additional hours of
treatment with 4 % CO/He flow (20 mL min 1) at the same temperature. Afterwards, the samples were evacuated at 100 8C at 10 4 mbar
for 30 minutes. CO and methanol in a molar ratio of 10:1 were
adsorbed at 100 8C. For mechanistic studies, time-resolved IR spectra
were recorded at increasing temperatures.
For 13C MAS NMR experiments, samples H-Mordenite and CuH-Mordenite were dehydrated at 400 8C overnight, treated with 13CO
(500 mbar) at 350 8C during 2 h, and subsequently degassed at 350 8C
for 1 h. Then, 13CO (2200 mmol per gram zeolite) and subsequently
13
CH3OH (730 mmol per gram zeolite), corresponding to 13CH3OH/
Al = 0.5, were introduced onto the activated zeolite, resulting in a
molar ratio 13CO/13CH3OH = 3. The glass inserts were sealed while
they were immersed into liquid nitrogen. To monitor the reaction,
NMR spectra were recorded following treatment of the sample at
200 8C during increasing reaction times outside the NMR probe.
Solid-state NMR spectra were recorded at room temperature with a
Bruker AV 400 WB spectrometer. The glass inserts were fitted into
7 mm rotors and were spun at 5 kHz in a Bruker BL7 probe. 13C MAS
NMR spectra were recorded with proton decoupling, with 908 pulse
length of 5 ms and a recycle delay of 30 s.
Received: January 3, 2007
Revised: February 28, 2007
Published online: April 17, 2007
Experimental Section
H-Mordenite was obtained by calcination of a commercial sample of
mordenite (ammonium form; Si/Al2 = 20; CBV20A from Zeolyst
International) at 500 8C during 2 h. Cu-H-Mordenite catalyst was
prepared by a two-step ion-exchange method as follows: first,
commercial mordenite (9.58 g) was mixed with an aqueous solution
(100 mL) of copper nitrate (1.507 g of Cu(NO3)2�H2O), and the
mixture was heated at reflux at 80 8C during 2 h. The slurry obtained
was filtered and dried at 100 8C for 12 h. Then, the resulting solid was
mixed again with an aqueous solution of copper nitrate solution
(0.78 g of Cu(NO3)2�H2O; 87 mL). The mixture was heated again at
reflux at 80 8C for 2 h and filtered, and the slurry was dried at 100 8C
4014
www.angewandte.de
.
Keywords: carbonylation � copper � heterogeneous catalysis �
reaction mechanisms � zeolites
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carbonylation, involving, activ, evidence, mechanism, zeolites, multisite, metalцacid, center, methanol
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